With the continuing development of sophisticated command, control, and communication data processing systems, spread spectrum communication techniques have drawn particular attention because of a number of advantages they offer over more conventional and limited bandwidth modulation schemes. One advantage is the capability of enabling the communication link to exhibit a robustness against jamming or natural interfering signals which are not correlated with the particular spreading waveform. These interference signals may include jamming, randomly distributed natural events, or other users of the same spectrum. A further advantage is that a signal-to-noise improvement may be obtained by systems which employ a plurality of codes (symbol alphabet) by transmitting a sequence of spread symbols whose energy distance has been maximized and equalized to enhance the decision thresholds as opposed to using an uncoded signal. In addition, enhanced time resolution may be obtained with the increased bandwidth.
The advantages of employing such a communication scheme are not obtained with ease, however, as part of the price one must pay is the complexity of the signal processing required to extract the useful information or data and the complexity of acquisition and synchronization processes needed. Typical modulation schemes employ coded sequences to define symbols which transmit each code bit of information, so that, at the receiver, some form of correlation or matched filtering, matched to each symbol of the symbol alphabet, is required to synchronize the receiver to the transmitter and extract the original data.
More specifically, as is well known, a signal waveform, or its complement, can be used to transmit a data symbol at some bit rate f.sub.B =1/TB on a carrier f.sub.c, with the bandwidth being determined by the duration of the chip T.sub.C =T.sub.B /N.sub.C (where N.sub.C is the number of chips per symbol) rather than symbol duration. This results in an increase in the bandwidth by a factor of N.sub.C yielding a spread spectrum signal. A conventional technique for detecting and decoding the data stream involves the use of a correlation receiver in which locally generated replicas of the symbol sequence are mixed with the incoming signal. Unfortunately, precise chip synchronization must be maintained, resulting in system complexity which makes this approach disadvantageous.
An alternate solution to data detection is the use of a matched filter receiver. In this case, different biorthogonal pseudo random sequences may be chosen to define the respective symbols in the symbol alphabet to be transmitted and, for a signal alphabet more complex than a binary one, a given number of information bits can be transmitted at a specified error rate with less total energy than is required for an optimum antipodal energy signal. In exchange for a savings in required bit energy, however, one must trade off increased bandwidth and equipment complexity.
In one such type of communication scheme, commonly termed M-ary transmission, an alphabet of M=2.sup.K symbols is defined, with the transmission of one symbol comprising log.sub.2 M=K bits of binary data. Each symbol consists of a sequence of n elementary signals or chips, with n&lt;&lt;K, typically, in order for each member of the alphabet to be orthogonal to all other members. The symbol spreading can be done in one phase (BPSK) by N chips or on two quadrature phases, (QPSK) N I-phase chips and N Q-phase chips, with or without staggering the I and Q chip transitions.
Optimal processing at the receiver requires the incoming signal to be correlated with one of M possible symbol waveforms. The receiver determines the most probable waveform to be that waveform having the highest degree of correlation as measured by the M receiver correlators. This decision minimizes the probability of error.
Previous attempts to manufacture a device for this purpose have included the CCD correlator, the optical Bragg cell correlator, the digital sum correlator, the SAW Fourier Transform correlator, the programmable SWD tapped delay correlator and the SWD airgap and elastic convolvers.
In operation, several of these correlators typically quantize and then delay the incoming signals for implementing the signal matching process. At the end of the symbol duration Ts a correlation pulse output is produced if the input signal matches a prescribed reference signal. Unfortunately, quantization of the input coded signal requires a high level of digital encoding in order to pass the wide dynamic range of input signals to be processed. This normally means that the input signal must be coded with from eight to twelve bits in order to provide adequate dynamic range. This increases the complexity of the hardware and substantially increases processing problems.
Moreover, these devices suffer from a number of other disadvantages such as large size, high power consumption, low delay stability, limited temperature range, high cost, low reliability, low dynamic signal range, high distortion and insertion loss, non-coherent output of the type most suitable for a multi-symbol coherent decoding process, and limited time-bandwidth products. As a result, these types of correlators are not suitable for systems whose signal formats require demodulation with ultra high speed PN and carrier phase acquisition.